1. Field of the Invention
The present invention relates to a constant-voltage circuit, and, in particular, to a constant-voltage circuit which generates a constant voltage from a power-source voltage.
2. Description of the Related Art
FIG. 1 shows an example of a circuit arrangement in the related art.
A constant-voltage circuit 11 in the related art includes a reference voltage generating circuit 12, a voltage detecting circuit 13, a comparing circuit 14, and a transistor Q11. The reference voltage generating circuit 12 generates a reference voltage Vref from a power-source voltage Vcc. The voltage detecting circuit 13 detects a detected voltage Vs in accordance with a constant output voltage Vout. The comparing circuit 14 compares the reference voltage Vref generated by the reference voltage generating circuit 12 and the detected voltage Vs detected by the voltage detecting circuit 13 with one another. The transistor Q11 draws current into the ground from an output terminal Tout in accordance with the comparison result of the comparing circuit 14.
The power-source voltage Vcc is lowered through a resistor R11 by a voltage drop and supplied to the reference voltage generating circuit 12, voltage detecting circuit 13, comparing circuit 14 and transistor Q11.
The reference voltage generating circuit 12 is connected between the resistor R11 and the ground GND,and includes a resistor R12 and a Zener diode Dz which are connected in series. The reference voltage Vref, which is the Zener voltage of the Zener diode Dz, is output from the connection point Pref between the resistor R12 and Zener diode Dz.
The voltage detecting circuit 13 is connected between the resistor R11 and the ground GND, and includes resistors R13 and R14 which are connected in series. The voltage, obtained as a result of the power-source voltage Vcc being lowered by the voltage drop, is divided, and, thus, the detected voltage Vs is output from the connection point Pvs between the resistors R13 and R14.
The reference voltage Vref generated by the reference voltage generating circuit 12 is supplied to the inverting terminal (-) of the comparing circuit 14, and the detected voltage Vs detected by the voltage detecting circuit 13 is supplied to the non-inverting terminal (+) of the comparing circuit 14. The comparing circuit 14 outputs the differential voltage (Vs-Vref) between the detected voltage Vs and the reference voltage Vref.
The output voltage of the comparing circuit 14 is supplied to the base of the transistor Q11. The transistor Q11 is an NPN transistor, the collector thereof being connected to the connection point between the resistor R11 and the output terminal Tout, and the emitter thereof being grounded.
The transistor Q11 increases its emitter current as a result of a rise of the output voltage of the comparing circuit 14. As a result, the current supplied to the output terminal Tout decreases. The transistor Q11 decreases its emitter current as a result of a fall of the output voltage of the comparing circuit 14. As a result, the current supplied to the output terminal Tout increases.
For example, when the power-source voltage Vcc rises, the output voltage Vout rises, and the detected voltage Vs detected by the voltage detecting circuit 13 also rises. As a result of the rise of the detected voltage Vs, the output voltage of the comparing circuit 14, which is the differential voltage (Vs-Vref) between the detected voltage Vs and the reference voltage Vref, thus rises.
As a result the rise of the differential voltage (Vs-Vref) of the comparing circuit 14, the base voltage of the transistor Q11 rises. Because the transistor Q11 is the NPN transistor, as a result of the rise of its base voltage, its emitter current increases, and thereby, the current drawn through the transistor Q11 into the ground GND increases. As a result of the increase of the current drawn through the transistor Q11 into the ground GND, the current which is supplied to a load (not shown in the figure) decreases. Thereby, the output voltage Vout falls.
By the above-described operations, the output voltage Vout is maintained to be constant.
When the power-source voltage Vcc falls, the output voltage Vout falls, and the detected voltage Vs detected by the voltage detecting circuit 13 also falls. As a result of the fall of the detected voltage Vs, the output voltage of the comparing circuit 14, which is the differential voltage (Vs-Vref) between the detected voltage Vs and the reference voltage Vref, thus falls.
As a result of the fall of the differential voltage (Vs-Vref) of the comparing circuit 14, the base voltage of the transistor Q11 falls. Because the transistor Q11 is the NPN transistor, as a result of the fall of its base voltage, its emitter current decreases, and thereby, the current drawn through the transistor Q11 into the ground GND decreases. As a result of the decrease of the current drawn through the transistor Q11 into the ground GND, the current which is supplied to a load (not shown in the figure) increases. Thereby, the output voltage Vout rises.
By the above-described operations, the output voltage Vout is maintained to be constant.
However, in the constant-voltage circuit in the related art, energy consumption occurs as a result of the current flowing through the Zener diode which generates the reference voltage, resistors which are used for detecting the output voltage, the comparing circuit which compares the reference voltage and the output voltage with one another, and the output controlling transistor Q11 which controls the output voltage as a result of bypassing the current in accordance with the output voltage of the comparing circuit.
Further, in the constant-voltage circuit in the related art, when the output voltage rises, the emitter current of the transistor increases. Thereby, the current supplied to the output terminal Tout is reduced. Thus, the output voltage Vout is maintained to be constant. Accordingly, as shown in FIG. 2, when the power-source voltage rises, the current (useless current) directly flowing to the ground GND increases. Thus, it is difficult to reduce current consumption.
For example, when the constant output voltage 1 V is obtained, approximately 1 .mu.A is consumed in the Zener diode, approximately 0.5 .mu.A is consumed in the resistors and comparing circuit, approximately 0.5 .mu.A is consumed in the output controlling transistor Q11. Thus, a total of 2 .mu.A is uselessly consumed.